Some comforting words for people experiencing a serious illness are "I am here if you need me," and "I love you." Genuine words of caring that express sentiments such as "You mean a lot to me," and "I'm sorry you have to go through this," are meaningful and comforting.
If looking for quotes to express comforting thoughts, a good one comes from Ralph Waldo Emerson, who stated "when it is dark enough, you can see the stars." Another nice sentiment comes from Beth Mende Conny, who reminds people that the future comes one moment at a time, which is helpful when the future feels overwhelming.
Some people wish to talk about their illness, while others do not. Many find it comforting when told they can make the choice to talk about the illness as long as they want to.